The present invention relates to an apparatus and method for YC separation and three-line correlation detection providing luminance (often represented as Y) signals with high accuracy in luminance and chrominance signals separation, i.e., YC separation using between-lines correlation found in composite video signals.
In recent years, three-line correlation detection has received much attention for its luminance and chrominance signals separation (YC separation) with high accuracy, which is effectively used in a cost-valued television-set having no three-dimensional YC separator with frame memory.
Now will be described an example of the prior-art three-line correlation detecting apparatus, referencing to the accompanying drawings.
FIG. 8 shows a block diagram of an YC separator employing the correlation detecting apparatus disclosed in Japanese Patent Laid-Open No. 8-65706. In the figure, receiving composite video signals as an input, three-line signal separator (three-line comb filter) 1 outputs a separated chrominance signal Cxe2x80x2. Band-pass filter (BPF) 3 isolates high-band components from the composite video signals to output a chrominance signal Cxe2x80x3.
According to the output from correlation detector 2, i.e., the output from OR circuit 9, selector 11 chooses either the signal Cxe2x80x2 fed from three-line comb filter 1, or the signal Cxe2x80x3 fed from BPF 3 filtering chrominance signals. Selector 11 then passes the selected signal CC to one end of subtractor 15 as the chrominance signal.
The composite video signal are also fed into delay circuit 13, which controls output timing by providing the signal with a delay, and then passed to the other end of subtractor 15. Receiving the delayed composite video signal from delay circuit 13, subtractor 15 subtracts signal CC from the delayed signal to generate luminance (Y) signal.
Here will be described the object of correlation detector 2 and the structure of the three-line correlation detecting apparatus both of which are introduced in the prior-art.
Suppose that processing the composite video signal having a high correlation in a direction perpendicular to the horizontal linesxe2x80x94the signal with a high vertical correlation with respect to the screenxe2x80x94for example, an image showing vertical stripes. In this case, allowing selector 11 to output signal Cxe2x80x2 fed from three-line C separator (three-line comb filter) 1 as signal CC to subtractor 15 can generate a Y signal with a good quality.
Now suppose that processing the composite video signal with a low vertical correlation with respect to the screenxe2x80x94for example, an image showing one horizontal red scanning lines against a white background. If a Y signal is generated from output signal Cxe2x80x2 determined as signal CC, dot interference caused by chrominance signals occurs at the horizontal red lines on the screenxe2x80x94a structural weak point of three-line comb filter 1. That is, because the chrominance level of output signal Cxe2x80x2 at the horizontal red lines is decreased to half its normal value, subtractor 15 cannot completely cancel out the chrominance signal. As a result, the residual chrominance signals in the Y signal cause dot interference. In such a screen with a low vertical correlation, allowing selector 11 to output signal Cxe2x80x3 fed from BPF 3 as signal CC can generate a Y signal, with dot interference from the chrominance signal suppressed. In this case, however, the high band characteristics of the Y signal are deteriorated.
As described above, the YC separator using the correlation detecting apparatus can properly switch between output signal Cxe2x80x2 and output signal Cxe2x80x3 according to the level of the detected vertical correlation with respect to the screen, which can generate a good Y signal.
FIG. 9 is a block diagram of the YC separation circuit that is embodied in Japanese Patent Laid-Open No. 8-65706. In the figure, frame 66 surrounded by the dotted lines represents the three-line correlation detecting apparatus, the rest in the figure shows the YC separator.
FIG. 10 shows a block diagram indicating the vertical impulse detector of three-line correlation detecting apparatus 66.
In FIG. 9, the composite video signals are separated into the 0H signal, the 1H signal (delayed by delayed element 21 for one horizontal scanning period), and the 2H signal (delayed by delayed elements 21 and 23 for two horizontal scanning periods), each of which is filtered by low-pass filters (LPFs) 41, 43, and 45, respectively. The filtered signals f, g, and hxe2x80x94the low-band components (luminance signals) of the composite video signal passed through LPFs 41, 43, and 45, respectivelyxe2x80x94are fed into low-band vertical impulse detector 47. On the other hand, high-band components (chrominance signals) of the composite video signal, which have passed through band-pass filters (BPFs) 49, 51, and 53, have opposite phases by 1H. Inverters 55 and 57 process the signals having different phases into in-phase chrominance signals i, j, and k, all of which are fed into high-band vertical impulse detector 59.
FIG. 10 shows the structure of the vertical impulse detector, which is employed for detector 47 for low-band and detector 59 for high-band. In the figure, accepting signals f, g, and h, subtractors 71 and 73 calculate differential signals by subtracting signal f from signal g, and by subtracting signal h from signal g, respectively. Absolute-value calculators (ABSs) 75 and 77 obtain each absolute value of respective differential signals. Receiving the two values, comparators 79 and 81 compare each value with respective predetermined reference values REFs, which are predetermined by comparators 79 and 81. The two outputs from comparators 79 and 81 are applied to AND circuit 83.
To provide the detection through the process above with accuracy, exclusive NOR circuit 85 is placed between the subtractor and ABS. If circuit 85 detects that the two differential signals have same signs, the output from circuit 85 and the output from AND circuit 83 are further applied to AND circuit 87, with the final output in FIG. 10 obtained.
High-band vertical impulse detector 59 shown in FIG. 9 can be the same as the structure illustrated in FIG. 10.
The output from detector 47 and the output from detector 59 are applied to OR circuit 61, and the result is determined as the output of three-line correlation detector 66. If vertical impulse is detected either detector 47 or 59, detector 66 determines that the correlation is low. The output from detector 66 takes the form of xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d: xe2x80x9c1xe2x80x9d indicating low correlation, xe2x80x9c0xe2x80x9d indicating the presence of the correlation.
As described above, the prior-art three-line correlation detecting apparatus detects correlation between the lines carrying the chrominance signal and the luminance signal of the composite video signal, and then outputs xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d depending on the presence or absence of the correlation.
According to the output from the correlation detecting apparatus, YC separator switches the filter used in separation; when accepted the output that represents the presence of the correlation, the separator uses three-line comb filter (5 tap median filter), otherwise uses BPF. In the case that a screen shows one horizontal red scanning lines against a white background described earlier, the correlation detector determines that the correlation is low, thereby uses BPF to generate the Y signal. This therefore suppresses dot interference in the Y signal. It still has, however, room for improvement in performancexe2x80x94a series of noises vertically generated on the screen.
The vertically generated in-series noises may occur between adjacent video processing devices. Compared to a noise occurred randomly, the noise spoils the view due to its occurrence in series on a regularly basis.
The frequency spectrum of such a noise is distributed over the range from the lower-middle band to high band of the luminance signal, especially the component of the high-band is to be an xe2x80x9ceyesorexe2x80x9d on the screen. In the YC separator, the Y signal generated through a comb filter has better high-band characteristics than that generated through a BPF. The fact makes noises to be conspicuous.
Because the vertically generated in-series noises have a vertical correlation, the detecting apparatus mistakenly determined that there is a correlation, accordingly the YC separator performs the separation through the comb filter. As a result, the misjudgment makes the noise conspicuous.
The present invention addresses the problem described above. It is therefore the object to provide an apparatus and method for YC separating and detecting correlation in order to accurately detect correlation between video signals, which is able to determine that the correlation is not so high as for the vertically generated in-series noises at the same time.
Now will be described the workings of the three-line correlation detecting apparatus of the present invention.
The apparatus includes a sub-correlation detector, by which input signals are sub-checked for the vertical correlation. From the sub-check, the characteristics of a signal is sub-determined to be xe2x80x9chaving a high-correlationxe2x80x9d or xe2x80x9cotherwisexe2x80x9d. Because the vertically in-series noises include jitter components and variations in amplitude, the sub-correlation detector is controlled to have a level of the threshold to get the determination of xe2x80x9cotherwisexe2x80x9d. If accepted the result of xe2x80x9chigh correlationxe2x80x9d from sub-checking, the three-line correlation detecting apparatus determines that the result can be reliable and outputs xe2x80x9chigh correlationxe2x80x9d signal preferentially. On the other hand, as for the signals sub-determined as xe2x80x9cotherwisexe2x80x9d, the apparatus increases the level of detecting accuracy and places the output in xe2x80x9cmxe2x80x9d levels according to the degree of the correlation. In this way, the vertically in-series noises are properly positioned in the m-leveled judgment.
The YC separator generates the Y signal according to the result fed from the three-line correlation detecting apparatus; i) generating the Y signal through the three-line comb filter for the signal xe2x80x9chaving a high correlationxe2x80x9d, ii) generating the Y signal through the BPF for the signal xe2x80x9chaving the least correlationxe2x80x9d, and iii) generating the Y signal by mixing the output from the comb filter with the output from the BPF, or by using a specific filter for the middle-ranged signal.
Since the three-line correlation detecting apparatus detects the vertical correlation in the chrominance signal, it is possible to detect the vertical correlation from the chrominance differential signal by isolating the chrominance signal from the composite video signal. Generally, however, the presence or absence of the vertical correlation in the chrominance signal and the luminance signal are closely related to each other. Therefore, providing the correlation detecting apparatus capable of detecting the vertical correlation of the two signals simultaneously contributes to higher accuracy in the detection.
Furthermore, incorporating the sub-correlation detector described earlier into the structure above can provide the correlation detecting apparatus with much higher accuracy and reliability.
With such structured correlation detecting apparatus, as described above, the YC separator allows to generate the Y signal with the interference minimized including dot interference, the vertically in-series noises and the noise interference of the signal having a middle-ranged vertical correlation. Thus, the practical method can provide the video images with high quality.
Basically structured the same as the three-line correlation detecting apparatus, the sub-correlation detector is primarily designed with the aim of detecting the signal having a high-correlation. From the purpose, detecting the impulse state in the differential signal between lines is not required to the sub-correlation detector; the required thing to the detector is to provide the output only if the differential signal has a small value. In addition, the sub-correlation detector has a lot common in the circuit design with the three-line correlation detecting apparatus. This allows them to share some circuits, realizing a low parts count.
The structure and workings of each component will be explained in detail in the description of the preferred embodiments of the present invention.